The present invention relates to a semiconductor laser device and a module having an optical fiber and, more particularly, to a module which is free from an influence of a reflected beam at an optical fiber end face and decreases a coupling loss.
When a compact semiconductor laser module, especially, a module using no refrigeration by a Peltier element is to be formed, the following scheme is used. That is, a semiconductor laser chip and a monitoring photodiode chip are sealed in a compact CAN package having a diameter of 5.6 mm in advance, and the resultant structure is screened as a semiconductor laser device. Thereafter, it is incorporated in a holder to which a lens is fixed, and the optical axis of an optical fiber is adjusted and fixed, thereby obtaining a module having the optical fiber.
FIG. 4 shows a conventional semiconductor laser module. A stem 3 having a diameter of 5.6 mm is popularly applied to a pick up semiconductor laser device for an optical disk and considered as one of the standards. This is one reason for popularly using the module structure shown in FIG. 4.
The semiconductor laser module shown in FIG. 4 has the following structure. In the structure, a lens 5 and an optical fiber 7 which are opposite to each other in an axial direction are fixed to a lens holder 6, and a semiconductor laser device 20 is fixed to the lens holder 6 so that axes 10 thereof coincide with each other.
In the semiconductor laser device 20, a semiconductor laser chip 1 is fixed to the stem 3 through a heat sink 2 for radiating heat generated by the semiconductor laser chip 1, and they are hermetically sealed by a cap 4 having a window. The light emitting position of the semiconductor laser chip 1 is on an axis 10 of the stem 3.
When a radiation beam of the semiconductor laser chip 1 is reflected by an optical fiber end face 7a and returned to a semiconductor laser cavity, a mode stability of the semiconductor laser becomes unstable to cause noise appearance for transmission. For this reason, the semiconductor laser module is generally designed to prevent the reflected beam at the optical fiber end face 7a from being returned to the semiconductor laser chip 1. In FIG. 4, the optical fiber end face 7a is polished at an angle of about 9.degree. . In this case, an optical beam radiated from the semiconductor laser chip 1 is focused by the lens 5 to form a beam waist. If the optical fiber end face 7a is positioned at the beam waist, the mode of the optical beam is converted into a mode which transmits through the optical fiber 7. At this time, although a reflected return beam is produced at the optical fiber end face 7a at a reflectivity of about 4%, since the optical fiber end face 7a has an inclination angle of about 9.degree. , the reflected return beam is incident at a large angle on a facet of the semiconductor laser chip 1. Therefore, the level of the beam coupled to the intrinsic mode of the semiconductor laser chip 1 is to be very low. For this reason, the operation state of the semiconductor laser chip 1 is not disturbed by the reflected return beam.
In the conventional semiconductor laser module, since the optical fiber end face 7a is inclined at an angle of about 9.degree. to suppress the influence of a reflected return beam, the beam which is incident on the optical fiber 7 is not parallel to the axis of the optical fiber 7, i.e., an excessive coupling loss is inevitable. In the semiconductor laser module in FIG. 4, the coupling loss is about 7 dB, 3 dB of the coupling loss correspond to an excessive loss caused by polishing the end face with an inclination. This is a major problem when a compact and high-output semiconductor laser module is to be formed.